1. Field of the Invention
The present invention relates to a pressure sensitive adhesive optical film having a pressure-sensitive adhesive layer laminated on at least one surface of the optical film. Specifically, it relates to an image viewing display using the pressure sensitive adhesive optical film, such as a liquid crystal display, an organic electro luminescence display, a PDP, and a CRT. As the optical film, an optical film obtained by laminating a polarizing film, a retardation film, an optical compensating film, a brightness enhanced film, and an antiglare sheet etc. may be mentioned.
2. Description of the Related Art
Conventionally, as optical films for liquid crystal displays (LCD), for example, a polarizing film, a retardation film, or an elliptical polarization film obtained by lamination thereof has been known. These optical films are used as a pressure sensitive adhesive optical film in which pressure sensitive adhesive layers, such as acrylic pressure sensitive adhesive layer, are beforehand formed for attachment on a liquid crystal cell, for the purpose of increase in efficiency of LCD assembly, prevention of variation in quality, etc.
In the above-mentioned pressure sensitive adhesive optical films, it is required to be excellent in
(1) reworkability (reproduction property): a property of reworking that an optical film can be peeled from a liquid crystal cell in case of wrong positioning in attaching or of admixing of foreign matter into laminated face in a process of attachment of a film to a liquid crystal cell, and
(2) moisture heat resistance of forming neither foaming nor peeling before and after being heat-treated in a state where it is mounted in an LCD. Recently, requirement for the above-mentioned performance has become higher in accordance with performance upgrading and upsizing of LCDs, leading to demand for improvement in durability sustaining still severer conditions. Moreover, since cost of a liquid crystal cell also increases in accordance with performance upgrading and upsizing of an LCD, demands for the reworkability (1) of an optical film enabling reproduction of an expensive liquid crystal cell are further increasing.
In conventional pressure sensitive adhesive optical films, in order to improve moisture heat resistance (2) in a state where it is mounted in an LCD, a measure of improving adhesive property between a pressure sensitive adhesive layer and a glass plate has been adopted. However, adhesive improvement in pressure sensitive adhesive layer increases a peeling force at time of removing an optical film from a liquid crystal cell. Therefore, a discrepancy causing damage of a glass plate for an LCD itself in case of reworking easily occurs, thus leading to deterioration in reworkability (1). Moreover, there is a tendency of decreasing in thickness of a glass plate of an LCD, which makes the glass plate more easily broken. On the other hand, design of a pressure sensitive adhesive layer placing a premium on a function of reworkability (1) worsens the moisture heat resistance (2). That is, reworkability (1) and moisture heat resistance (2) are in a relationship of trade-off.
Furthermore, in a pressure sensitive adhesive optical film, it is required to be excellent in (3) optical function retentivity that neither transmittance of light nor an optical property of retardation (a retardation value: a product of a refractive-index difference Δn of birefringence and a film thickness d) etc. falls even after kept in a high temperature ambient atmosphere. However, in general, there is a tendency that adhesive power improvement in a pressure sensitive adhesive layer for improving the moisture heat resistance (2) raises an elastic modulus of the pressure sensitive adhesive layer. And, improvement in an elastic modulus causes thermal expansion difference between adherends, such as liquid crystal cell, etc., and then increase of a stress of an optical film, which deteriorates optical function retentivity (3), as a result causes deterioration of an optical property of the optical film by heating. Especially when a liquid crystal cell is kept in a high temperature ambient atmosphere, display irregularity will arise on periphery of the liquid crystal cell. On the other hand, although the display irregularity on the periphery shows a tendency of being improved by lowering an elastic modulus of the pressure sensitive adhesive layer, in that case, moisture heat resistance (2) decreases.